1. Field of the Invention
The present invention relates to an anti-theft vehicle security system having a function for making it impossible to start an engine when an attempt is made for driving the motor vehicle in an unauthorized way for the purpose of stealing.
2. Description of the Related Art
In general, a possible case of stealing a vehicle includes a direct connection to an engine starting circuit (starter power supply) or use of a fake engine key, etc. Anti-theft security systems such as one disclosed in Japanese Patent Laid-Open Publication No. 6-227364 are known devices for preventing this type of theft. Noting the fact that an increasing number of new vehicles employ methods of controlling the fuel amount or ignition timing of the engine based on an engine control device, the anti-theft security system determines whether the engine key used in starting the engine is a proper key or not based on an ID code contained in the engine key. Based on the result of such determination, data as to whether starting of the engine should be allowed or not is transmitted to the engine control device by an electronic means, thereby the fuel supply and ignition timing of the engine are controlled to start the engine.
In the case as described above where the starting of the engine begins based on the result of an authentication as to whether the engine key is a proper one or not, each device performs an initialization for starting its operation in accordance with an ON operation of the engine key and enters its signal communication and signal processing state. Then, an ID code which has been electronically read out from the engine key by a code read means provided in the vicinity of the key switch is transmitted to an anti-theft security device on the vehicle. The anti-theft security device generates an engine start affirmation/negation signal based on the result of collating of the transmitted ID code and a previously set proper code.
On the other hand, upon entering its signal processing state, the engine control device performs exchange of data with the anti-theft security device through a communication line to invoke transmission of data as to whether starting of the engine should be allowed or not in response to the invocation by the engine control device, the anti-theft security device outputs the engine start affirmation/negation data to the engine control device through the described communication line described above.
Upon determining that starting of the engine is permitted based on the engine start affirmation/negation data, the engine control device outputs a start signal to the engine starting circuit, thereby starting of the engine becomes possible. However, if the engine control device has determined that starting of the engine should not be allowed, no start signal is output to the engine starting circuit. As a result, the vehicle will not be stolen even when the engine key is counterfeited.
As described above, the anti-theft security device and engine control device in the conventional system perform exchange of data for determining whether starting of the engine should be allowed or not by means of a shared single communication line. Depending on the operation environment (e.g., at the time of very low temperature or when a degraded battery is used) in starting the engine, it may thus take time to initiate a normal exchange of data, resulting in a problem that starting of the engine is delayed corresponding to such time period.
An adverse operation environment at the time of starting the engine leads to a temporary drop in the battery voltage, resulting in a state in which securing an operation voltage of the communication line is difficult. In such state, if each device performs respective data communication, normal transmission is hampered and data may not be accurately transmitted to a receiving end. In addition, since a protocol corresponding to degraded state of communication is not defined in the conventional system, time is required to eliminate the degraded communication state. Thus data exchange time for starting the engine is prolonged so that the starting of the engine is delayed corresponding to the prolonged time.
Further, while operations of both the anti-theft security device and the engine control device are reset when the battery voltage drops starting the engine, the reset voltage, reset release voltage and reset release timing for each device are different from each other. As a result, start timing of data exchange is different between the respective devices, causing a communication conflict on the communication line between the devices. Further, a protocol for eliminating a communication conflict, or for coping with a detected conflict, is not defined in the conventional system. If, therefore, communication is continued in the state where the two devices are in a communication conflict, data exchange time for starting the engine is prolonged until the communication conflict is eliminated in some way. Thus, the starting of the engine is delayed corresponding for such prolonged time period.